Create a custom cache backend (cachem-like)

Builds a minimal cachem-like cache backend object that exposes cachem-compatible methods: $get(key, missing), $set(key, value), $remove(key), and $info().

Use this helper when you want to plug a custom state store or JWKS cache into 'shinyOAuth', when cachem::cache_mem() is not suitable (e.g., multi-process deployments with non-sticky workers). In such cases, you may want to use a shared external cache (e.g., database, Redis, Memcached).

The resulting object can be used in both places where 'shinyOAuth' accepts a cache-like object:

• OAuthClient@state_store (requires $get, $set, $remove; optional $info)

• OAuthProvider@jwks_cache (requires $get, $set; optional $remove, $info)

The $info() method is optional, but if provided and it returns a list with max_age (seconds), shinyOAuth will align browser cookie max-age in oauth_module_server() to that value.

Usage

custom_cache(get, set, remove, take = NULL, info = NULL)

Arguments

get

A function(key, missing = NULL) -> value. Required. Should return the stored value, or the missing argument if the key is not present. The missing parameter is mandatory because both OAuthClient and OAuthProvider validators will pass it explicitly.

set

A function(key, value) -> invisible(NULL). Required. Should store the value under the given key

remove

A function(key) -> any. Required.

Deletes the entry for key. When $take() is provided, $remove() serves only as a best-effort cleanup and its return value is ignored. When $take() is not provided, 'shinyOAuth' falls back to $get() + $remove() followed by a post-removal absence check via $get(key, missing = NA). In this fallback path the return value of $remove() is not relied upon; the post-check is authoritative.

take

A function(key, missing = NULL) -> value. Optional.

An atomic get-and-delete operation. When provided, 'shinyOAuth' uses $take() instead of separate $get() + $remove() calls to enforce single-use state consumption. This prevents TOCTOU (time-of-check / time-of-use) replay attacks in multi-worker deployments with shared state stores.

Should return the stored value and atomically remove the entry, or return the missing argument (default NULL) if the key is not present.

If your backend supports atomic get-and-delete natively (e.g., Redis GETDEL, SQL DELETE ... RETURNING), wire it through this parameter for replay-safe state stores.

When take is not provided and the state store is not a per-process cache (like cachem::cache_mem()), 'shinyOAuth' will error at state consumption time because non-atomic $get() + $remove() cannot guarantee single-use under concurrent access in shared stores.

info

Function() -> list(max_age = seconds, ...). Optional

This may be provided to because TTL information from $info() is used to align browser cookie max age in oauth_module_server()

Value

An R6 object exposing cachem-like $get/$set/$remove/$info methods

Examples

mem <- new.env(parent = emptyenv())

my_cache <- custom_cache(
  get = function(key, missing = NULL) {
    base::get0(key, envir = mem, ifnotfound = missing, inherits = FALSE)
  },

  set = function(key, value) {
    assign(key, value, envir = mem)
    invisible(NULL)
  },

  remove = function(key) {
    if (exists(key, envir = mem, inherits = FALSE)) {
      rm(list = key, envir = mem)
    }
    invisible(NULL)
  },

  # Atomic get-and-delete: preferred for state stores in multi-worker
  # deployments to prevent TOCTOU replay attacks. For per-process caches
  # (like cachem::cache_mem()) this is optional; for shared backends (Redis,
  # database) it should map to the backend's atomic primitive (e.g., GETDEL).
  take = function(key, missing = NULL) {
    val <- base::get0(key, envir = mem, ifnotfound = missing, inherits = FALSE)
    if (exists(key, envir = mem, inherits = FALSE)) {
      rm(list = key, envir = mem)
    }
    val
  },

  info = function() list(max_age = 600)
)